1. Field of the Invention
This invention relates to a Data Compression Method and a Data Transfer System using the data compression method.
2. Description of the Prior Art
It is quite common to transfer an image data after compressing it when a steady image or moving images are transferred electrically. The compression techniques are categorized in two methods, the reversible method such as GIF (Graphics Interchange Format) method and PNG (Portable Network Graphics) method and the irreversible method such as JPEG (Joint Photographic Coding Experts Group) method and MPEG (Moving Picture Coding Experts Group) method.
The compression rate is higher in the irreversible method than in the reversible one so that very often the irreversible method is employed when the data size is big and it is not necessary to recover the original data. The data size after the compression is dependent upon the image when employing the irreversible method. The irreversible method has a tendency, in general, to have a lower compression rate when color variation is large in an image. FIG. 7 shows an example of a compression method by classifying color. A choice of color is among 65,536 different colors upon the computer display when ‘Full Color’ is chosen. It is, therefore, necessary to deal with 65,536 different colors to compress at most. On the contrary, the compression rate becomes higher if the number of colors is minimized. Therefore, a mono-colored image ends up with a high compression rate and a colorful image ends up with a lower compression rate. Thus, the data size after compression is variable.
As a data culling process by a simple calculation taking in consideration the nature of human optical behavior, the portion of culled data has the feature that the averaged brightness data of 4 pixel points taken by the computer became the brightness data of the new pixel point after culling data. The color difference value of the new pixel point after culling data is preventively assigned to be the predefined color difference value based upon the color difference values of the 4 pixel points. (Laid-Open Application No. JP8-340552 (Page 4 to 5 and FIGS. 1-2)).
While transferring a large data without demanding reversibility, there may be two separate demands. The first demand may be the case the image quality is important and the transfer speed is important. For the irreversible compression technique the compression rate can be defined by the characteristic of the image such as, for example, the number of colors in the image even when data reappearability is taken into consideration. There is a case that the reappearability of compressed data is not important, in other words, when the number of colors to reappear is not large. For example, most LCD projectors have 3 LCD units for light modulation to handle 3 colors, Red, Green and Blue. It is also common that the digital-analog transformer that defines the tone of color handles 256 colors. Moreover, it is also common for a color printer not to reappear ‘Full Color’ for an image. Data communication between the two systems have different image resolutions, for example from a personal computer having a display capability of the XGA (extended Graphics Array) standard to a LCD projector having SVGA (Super Video Graphics Array) display capability, it is profitable to cull pixel points before compression because the XGA standard has more pixel points than the SVGA standard.
It takes sometime for data transfer while having a presentation sending data to an LCD projector through a radio data transfer system. If the data transfer time is constant, the presentation is performed with the certain interval taking into account the data transfer time and the time until the display shows up. It is necessary to have an automated compression system to approximate the compression rate without dependence upon the nature of the image.
It is known that the error rate increases while the transferring data rate using wired or radio communication system increases. Once a data error occurs, the portion of broken data has to be corrected by sending the same data set. The error rate, however, remains low if the quantity of data is small.
Data transfer by radio often uses mono frequency and a half duplex protocol so that the data transfer is asynchronous in general. Sending data asynchronously take more time than sending it synchronously because asynchronous does not go high on the data transfer rate than synchronous. A high data transfer time increases the chance for the data to be perturbated by noise and/or interference. The quantity of data for each transmission is preferably short and the equal amount for the point of view to employ the packet communication method.